He was walking home drunk, got jumped, and instead of playing it cool he ate pavement and went into a ditch. The water had leeches. That's the whole origin. Not a prophecy. A bad night.

An old man found him half-dead and hauled him into an oni village. He'd lost too much blood, so they put oni blood in him. He was dying. That's it.

He woke up with horns, panicked, and snapped the right one off. They tried to turn him back. The ritual didn't finish.

So he's half. There's an ofuda on the horn that's left so he doesn't flip mid-stream. Meat still sets him off. He lives on fruit. That's not a bit. That's the diet.